Medulloblastoma, the most common pediatric brain cancer, is a primitive neuroectodermal tumor (PNET) that arises in the cerebella. Patients with a disseminated phenotype and relapse have particularly poor prognoses. The Ptch+/-;p53-/- mouse model of medulloblastoma have been characterized as having a disseminated phenotype and contain a population of medulloblastoma stem cell (MBSC), which have the ability for self-renewal, expression of neural stem cells markers and potent tumor-initiation upon transplantation. The overall goal of this project is to undercover new molecular mechanisms that are important for the survival and maintenance of self-renewal in the medulloblastoma stem cells. This aims in this project will the test the hypothesis that the MBSC have a cellular plasticity that confers the cells a survival advantage and resistance to differentiation in non-niche environments, to support tumor dissemination. Preliminary results showed that the MBSC exhibit a cellular plasticity allowing them to survive and self-renew in the presence of the differentiation cues of serum. To gain a broader understanding of the pathways involved in the survival and plasticity, I will first determine the gene expression profile of the MBSC and the rol of STAT3 in the MBSC. Examination of LIF signaling, a known stem cell pathway, has led to the observation of aberrant STAT3 activation in the MBSC. The functional role of STAT3 in MBSC survival and self-renewal will be validated by RNAi and STAT3 small molecule inhibitors. Preliminary data also suggest that STAT3 and its downstream target Klf4, are collaboratively regulating the expression of survival genes Mcl-1 and Bclx. In the second aim, the cooperation of STAT3 and Klf4 in survival gene regulation will be tested by double KD of STAT3 and Klf4, as well as survival gene promoter-luciferase assays with mutant binding sites. Lastly, the therapeutic value of STAT3/Klf4 pathway inhibition will be tested with intracranial transplantation assays and to determine if a synergistic effect on MBSC elimination is observed with combination treatment with irradiation therapy and STAT3/Klf4 inhibition. Completion of the research proposal will provide insights into the cooperative role of STAT3 and Klf4 in MBSC survival and address the potential of STAT3/Klf4 inhibition as a therapeutic target for disseminated human medulloblastomas.